A plastic optical fiber (hereinafter, referred to as an “optical fiber” as appropriate) has been practically used for short-range communication applications such as lighting, FA, OA and LAN because of its advantageous characteristics such as a lower cost, a light weight, flexibility and a larger diameter although its transmission distance is shorter than that of a quartz optical fiber. Most of plastic optical fibers practically used to date have a core-clad structure with a core made of polymethyl methacrylate (PMMA), and have recently been expected to be used as a high-speed communication media in combination with a visible red light source operable with a high speed.
Technique for expanding a band in an optical fiber with a core-clad structure has been disclosed in JP-A 7-239420 and WO 96/36894, where a band in an optical fiber is expanded by reducing a difference in a refractive index between a core and a clad to reduce a numerical aperture in the optical fiber (NA reduction). The optical fibers described in these publications have improved a transmission band at a transmission distance of 50 m to 200 MHz by reducing a numerical aperture to about 0.3.
Various clads have been proposed for such an optical fiber. For example, JP-As 7-239420, 9-101423 and 9-159844 have disclosed a clad made of 1 to 30 wt % of a long-chain fluoroalkyl methacrylate, 1 to 20 wt % of a short-chain fluoroalkyl methacrylate and 50 to 98 wt % of methyl methacrylate and having an MI value of 5 to 60 g/10 min. JP-A 11-133252 has disclosed a clad made of a long-chain fluoroalkyl methacrylate and methyl methacrylate. JP-A 10-221543 has disclosed a clad made of 10 to 40 mol % of a short-chain fluoroalkyl methacrylate and 60 to 90 mol % of methyl methacrylate. These clads, however, exhibit inadequate strength.
On the other hand, as an optical fiber with a clad exhibiting higher strength, JP-As 8-101316 and 10-104455 have disclosed an optical fiber with a clad made of a vinylidenefluoride polymer or polymer mixture. When producing an optical fiber with a low numerical aperture using such a clad, a vinylidenefluoride polymer is used as a mixture with polymethyl methacrylate (PMMA) because the polymer itself has a low refractive index. However, such an optical fiber has drawbacks such as a significantly reduced transparency and increase in a transmission loss when bending the optical fiber.
As a technique for improving strength of a clad made of a fluoroalkyl methacrylate polymer, JP-Bs 7-11604 and 7-11605 and JP-As 1-76003, 1-105205, 1-223104, 3-062809 and 4-051206 have disclosed that a 2-(perfluorooctyl)ethyl methacrylate unit (referred to as “17FM”) as a rubber component is added to a copolymer for a clad to improve strength of the clad. The 17FM unit, however, reduces a refractive index of the copolymer. Therefore, when a copolymer contains the 17FM unit at a larger amount for improving strength of the clad, a refractive index of the copolymer is reduced so that an optical fiber using the copolymer as a clad may have an increased numerical aperture.
Thus, strength of a clad has not been adequately improved in an optical fiber with a lower transmission loss and a lower numerical aperture. There has been, therefore, a problem that when holding such an optical fiber as a bobbin-wound form common in storing or delivery, a transmission loss of the optical fiber is increased.